This invention relates generally to a coating composition and more particularly concerns a coating composition for the interior surface of a can.
It is well known that an aqueous solution in contact with an untreated metal substrate can result in corrosion of the untreated metal substrate. Therefore, a metal article, such as a metal closure or container for a water-based product, like a food or beverage, is rendered corrosion resistant in order to retard or eliminate interactions between the water-based product and the metal article. Generally, corrosion resistance can be imparted to the metal article, or to a metal substrate in general by coating the metal substrate with a corrosion-inhibiting coating.
Coating compositions for the inside of beer, beverage and food cans must be approved for direct food contact. The fundamental function of the inside coatings on cans and ends is to protect the packed product to maintain its nutritional value, texture, color and flavor when purchased and used by the consumer. To meet these requirements, the organic film must be free of any material which might extract into the packed product and must maintain its integrity over the product recommended shelf life. The ingredients in the can must not make contact with the metal surface. On the whole, current commercial coating compositions for cans and ends, have met all these performance criteria. The precoated metal sheets are subjected to severe elongative and compressive stresses during the can forming process of the fabrication of can ends. The integrity of the respective coating must be maintained during all the specific fabrication operations. The easy open ends, for two-piece beverage cans for example, are stamped from precoated aluminum coil and are fed through a series of presses where the rivet, scaring and tab making steps occur. Coatings for ends must be highly flexible to withstand this series of operations.
Investigators continually have sought improved coating compositions that reduce or eliminate corrosion of a metal article and that do not adversely affect an aqueous product packaged in the metal article. However, improving one particular advantageous property is often achieved at the expense of another advantageous property. Investigators have sought coatings that possess chemical resistance in addition to corrosion inhibition. A useful coating for the interior of a metal closure or container must be able to withstand the solvating properties of the packaged product. If the coating does not possess sufficient chemical resistance, components of the coating can be extracted into the packaged product and adversely affect the product. Even small amounts of extracted coating components can adversely affect sensitive products by imparting an off-taste to the product.
Epoxy resins are widely used in coating compositions for application to the interior surfaces of food cans. Bisphenol A diglycidyl ether (Badge) is widely used as a monomer for epoxy resins. When epoxy resins made from bisphenol A diglycidyl ether are employed in coating compositions that are used on the interior surfaces of food cans, residual bisphenol A diglycidyl ether can be extracted into the food, and for this reason, epoxy resins containing bisphenol A diglycidyl ether and used for this application have come under scrutiny by various regulatory agencies. Thus, it is highly desirable to develop a polyester resin that can be cross-linked by a hardener to provide an alternative to the currently employed combination of an epoxy resin and a crosslinking hardener.
Polyester resins possess many of the essential properties such as flexibility, adhesion to metal and corrosion resistance, that are needed in a resin that is employed in a can coating for the interior surface of a can, and additionally are free of bisphenol A diglycidyl ether. When cross-linked with suitable hardeners such as a phenoxy group-containing or amino group-containing resin, polyester resins can form excellent coatings. However, the critical shortcoming of conventional polyester resins for use in coatings of the interior surfaces of cans is that their ester bonds are susceptible to hydrolysis, especially during the retort process after the food is canned.
Japanese laid open patent application (Kokai) No. 4-39370 which was laid open on Feb. 10, 1992 discloses a resin composition for can coating materials characterized in that a polyester resin (A) and an alkyl etherified amino formaldehyde resin (B) are compounded in a proportion of (A)/(B)=95-60/5-40 (weight ratio) in which the dicarboxylic acid component is 20 to 80 mol % of isophthalic acid, 80 to 20 mol % of aromatic dicarboxylic acid such as naphthalene dicarboxylic acid other than isophthalic acid, 0 to 50 mol % of aliphatic dicarboxylic acid of 2 to 12 carbon atoms, the glycol component is 25 to 99 mol % of glycol containing at least one or two or more side chain alkyl groups as indicated by general formula (I) below, 1 to 75 mol % of glycol with 8 to 12 carbon atoms and 0 to 50 mol % of other alkyl glycols, and, in which, further, polycarboxylic acids and/or polyols that are trivalent or of higher valence are present in an amount of 0.5 to 10 mol % relative to the total acid component. 
wherein, R1 and R2 are alkyl groups with 1 to 6 carbon atoms. However, the resin composition is disclosed as being applicable as a coating for the exterior surface of a can.
It is therefore a general object of the present invention to provide an improved coating composition for the interior surface of a can which solves the aforementioned problem and meets the aforementioned need.
More particularly, it is an object of the present invention to provide an improved coating composition for the interior surfaces of a can which employs a polyester resin cross-linked with a phenoxy group-containing or amino group-containing resin that is stable to hydrolysis or sterilization.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims.
The aforesaid objects are achieved by the coating composition of the present invention for application to an interior surface of a can, comprising a phenoxy group-containing or amino group-containing resin cross-linking agent in an amount in the range of from about 2 to about 45 weight percent of the coating composition, a polyester resin in an amount in the range of from about 6 to about 59 weight percent of the coating composition, and a nonaqueous carrier in the range of from about 35 to about 80 weight percent of the coating composition, wherein the polyester resin comprises a condensation product formed from (a) from about 2 to about 48 mole percent of isophthalic acid, (b) from about 2 to about 48 mole percent of a naphthalene dicarboxylic acid or alkyl ester thereof, (c) from about 0 to about 10 mole percent of trimellitic anhydride or trimethylolpropane, and (d) from about 30 to about 60 mole percent of neopentyl glycol.
A coating composition of the present invention, after curing, provides a cured coating composition that effectively enhances corrosion inhibition of metal substrates, such as, but not limited to, aluminum, iron, steel and copper. The present coating composition, after curing, also demonstrates excellent adhesion to the metal substrate, excellent chemical resistance and scratch resistance, excellent flexibility, and excellent stability to hydrolysis and sterilization.
In general, the coating composition of the present invention for application to an interior surface of a can comprises a phenoxy group-containing or amino group-containing resin that serves as a cross-linking agent in an amount in the range of from about 2, preferably from about 4, more preferably from about 8, to about 45, preferably to about 40, more preferably to about 33 weight percent of the coating composition, a polyester resin of this invention in an amount in the range of from about 6, preferably from about 8, more preferably from about 10 to about 59, preferably to about 52, more preferably to about 39 weight percent of the coating composition, and a nonaqueous carrier in the range of from about 35 to about 80 weight percent of the coating composition.
A polyester resin of this invention comprises a condensation product formed from (a) from about 2, preferably from about 10, more preferably from about 25, to about 48, preferably to about 40, more preferably to about 38, mole percent of isophthalic acid, (b) from about 2, preferably from about 4, more preferably from about 8, to about 48, preferably to about 20, more preferably to about 15, mole percent of a naphthalene dicarboxylic acid or alkyl ester thereof, (c) from about 0, preferably from about 2, more preferably from about 4, to about 10, preferably to about 8, more preferably to about 7 mole percent of trimellitic anhydride or trimethylolpropane, and (d) from about 30, preferably from about 42, more preferably from about 44, to about 70, preferably to about 60, more preferably to about 56, mole percent of neopentyl glycol. The polyester resin of the present invention is free of bisphenol A diglycidyl ether.
The polyester has a molecular weight of about 1,000 to about 50,000, and preferably about 1,000 to about 10,000. To achieve the full advantage of the present invention, the polyester has a molecular weight of about 1,500 to about 6,000. The polyester is prepared by methods well known in the art. The esterification reaction is preferably carried out in a nitrogen atmosphere at temperatures of between 180xc2x0 C. and 260xc2x0 C. Catalysts such as dibutyltin oxide, tin-chloride or tetrabutoxy titanate, and antioxidants such as triphenyl phosphite may be used as additives. The reaction water released during the reaction is removed by distillation. The desired degree of esterification is reached by means of azeotropic distillation or vacuum in the last phase. The reaction results in a polyester that can subsequently be dissolved or emulsified in water, an organic solvent or a mixture of solvents. Suitable solvents include aromatic hydrocarbons such as, for example, Solvesso 150(trademark) (Esso), esters of glycol ethers, such as, for example, methoxypropyl acetate (Dowanol PMA) or glycol ethers, such as, for example ethylene glycol monobutyl ether (Dowanol EB). The solids content of the polyester can, for example, range between 45 and 80% and is preferably between 50 and 65%.
The polyester has to be crosslinked with a hardener or crosslinker such as, for example, an amino group containing resin, a phenoxy group containing resin, or a phenolic resin. The weight ratio polyester:hardener resin is generally between 95:5 and 60:40 (based on solid resin). A suitable catalyst for the curing reaction includes, for example, dodecyl benzene sulphonic acid, phosphoric acid, or partial esters of phosphoric acid.
The present coating composition is a nonaqueous composition, wherein the cross-linking agent resin and the polyester are homogeneously dispersed in a nonaqueous carrier. In general, the nonaqueous carrier has sufficient volatility to evaporate essentially entirely from the coating composition during the curing process, such as during heating at about 170xc2x0 C. to about 225xc2x0 C. for about 8 to about 20 minutes. Suitable nonaqueous carriers are known in the art of coating compositions, and include for example, but are not limited to, glycol ethers, like ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, and propylene glycol monomethyl ether; ketones, like cyclohexanone, ethyl aryl ketones, methyl aryl ketones and methyl isoamyl ketone; aromatic hydrocarbons, like toluene, benzene and xylene; aliphatic hydrocarbons, like mineral spirits, kerosene and high flash VMandP naphtha; alcohols, like isopropyl alcohol, n-butyl alcohol and ethyl alcohol; and aprotic solvents, like tetrahydrofuran; chlorinated solvents; esters; glycol ether esters, like propylene glycol monomethyl ether acetate; and combinations thereof.
The nonaqueous carrier usually is included in the composition in a sufficient amount to provide a composition including from about 20% too about 50%, by weight of the composition, of the total weight of the polyester component. The amount of nonaqueous carrier included in the composition is limited only by the desired, or necessary, rheological properties of the composition. Usually, a sufficient amount of nonaqueous carrier is included in the coating composition to provide a composition that can be processed easily and that can be applied to a metal substrate easily and uniformly, and that is sufficiently removed from the coating composition during curing within the desired cure time.
Therefore, essentially any nonaqueous carrier is useful in the present coating composition as long as the nonaqueous carrier adequately disperses and/or solubilizes the composition components; is inert with respect to interacting with composition components; does not adversely affect the stability of the coating composition or the ability of the corrosion-inhibiting coating to inhibit corrosion of a metal substrate; and evaporates quickly, essentially entirely and relatively rapidly to provide a cured coating composition that inhibits the corrosion of a metal substrate, demonstrates good adhesion and flexibility, and has good chemical and physical properties.
A coating composition of the present invention also can include other optional ingredients that do not adversely affect the coating composition or a cured coating composition resulting therefrom. Such optional ingredients are known in the art, and are included in a coating composition to enhance composition esthetics; to facilitate manufacturing, processing, handling and application of the composition; and to further improve a particular functional property of a coating composition or a cured coating composition resulting therefrom.
Such optional ingredients include, for example, catalysts, dyes, pigments, extenders, fillers, lubricants, anticorrosion agents, flow control agents, thixotropic agents, dispersing agents, antioxidants, adhesion promoters, light stabilizers, and mixtures thereof. Each optional ingredient is included in a sufficient amount to serve its intended purpose, but is included in a sufficient amount so as not to adversely affect a coating composition or a cured coating composition resulting therefrom.
One useful optional ingredient is a lubricant, like lanolin, which facilitates manufacture of metal closures by imparting lubricity to sheets of coated metal substrate. A lubricant is present in the coating composition in an amount of 0% to about 2%, and preferably about 0.1% to about 2%, by weight of nonvolatile material. Another useful optional ingredient is a pigment, like titanium dioxide. A pigment is present in the coating composition in an amount of 0% to about 50%, and preferably about 10% to about 50%, by weight of nonvolatile material. In accordance with an important feature of the present invention, the present coating composition is free of a bisphenol A diglycidyl ether component.
A coating composition of the present invention is prepared by simply admixing the cross-linking agent resin, the polyester and any optional ingredients, in any desired order, in the nonaqueous carrier, with sufficient agitation. The resulting mixture is admixed until all the composition ingredients are homogeneously dispersed throughout the nonaqueous carrier. Then, an additional amount of the nonaqueous carrier can be added to the coating composition to adjust the amount of nonvolatile material in the coating composition to a predetermined level.